Existing internal combustion engines produce elevated level of emissions during transient loads (as compared to steady-state operation). This is because the transient loads may change faster than the engine can respond, which in turn, can cause a drop in the air-fuel ratio (in a load increasing situation). Such a drop in the air-fuel ratio increases the temperature of the exhaust, thus generating higher levels of NOx. The lowered air to fuel ratio also decreases the combustion efficiency which in turn produces higher level of particulate matter. If an independent power source of the hybrid system were to assist the engine during transient loads, the engine may be more likely to keep up with the changing load, thereby preventing the air-fuel ratio to drop below a desired level.
A hybrid system that utilizes an independent power source for assisting the engine during transient loads is disclosed in U.S. Pat. No. 7,152,705 (the '705 patent) issued to Alster et al. on Dec. 26, 2006. The system disclosed in the '705 patent includes two internal combustion engines, two generators, and a power storage device. During acceleration conditions, energy from the power storage device is used to assist the engines so that the engines can operate at their peak efficiency during acceleration.
Although the system disclosed in the '705 patent may utilize an independent power source (the power storage device) to assist the engines during acceleration events, the system's efficiency may be limited. In particular, the '705 system focuses on minimizing the fuel consumption of the engines for determining when and how much power is to be distributed from the power storage device. However, when an engine operates below its saturation point (i.e., its fuel limit), operating the engine without assistance from the power storage device may be more efficient, even when the engine is not operating at peak efficiency. This is because, due to various inefficiencies, only a fraction of the power transmitted to the power storage device is actually stored. When an engine operates below its saturation point, more energy might be needed to produce the power assisting the engine than would be needed by the engine to produce the same output without assistance. Therefore, the system disclosed in the '705 patent may not achieve a maximum efficiency when the engines operate below their saturation points during acceleration events.
The disclosed system is directed to overcoming one or more of the problems set forth above.